A varistor is an electrical component in which the current increases markedly as the voltage applied across the device increases. This characteristic makes the device suitable for applications such as protection against overvoltage surges in electrical circuits. Several types of surge suppressors are available, including:
Zener or avalanche diodes which are effective in clamping transients to low voltages but are costly to fabricate for high surge energy applications.
Metal oxide varistors based on zinc oxide or other metal oxides and fabricated by ceramic processing techniques. These devices are inexpensive to fabricate but operate best at high voltages and are difficult to adapt for low voltage (3 to 30 V) applications.
Various voltage-dependent resistors have been widely used for stabilization of voltage of electrical circuits or suppression of abnormally high voltage surges induced in electrical circuits. The electrical characteristics of such voltage-dependent resistors are expressed by the relation: EQU I=(V/C).sup.n
where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation: EQU n=[log.sub.10 (I.sub.2 /I.sub.1)]/[log.sub.10 (V.sub.2 /V.sub.1)]
where V.sub.1 and V.sub.2 are the voltages at given currents I.sub.1 and I.sub.2, respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the extent to which the resistors depart from ohmic characteristics.
Metal oxide varistors are usually manufactured by mixing a plurality of additives with a powdered metal oxide, commonly zinc oxide. Typically, four to twelve additives are employed. The metal oxide and additive mixture is then pressed into a body of a desired shape and size. The body is then sintered for an appropriate time at a suitable temperature as is well known in the prior art. Sintering causes the necessary reactions among the additives and the metal oxide and fuses the mixture into a coherent pellet. A passivating coating is sometimes applied to the sintered body. If a coating is applied, the body with the coating is generally reheated. Next, metallic contacts are applied to the body. The contacts can, for example, be applied by techniques such as the application of a silver paste or by metallic flame spraying.
A problem encountered in metal oxide varistors manufactured by the prior art method has been contact failure. Occasionally a contact will develop a crack or tear near the lead attachment or will peel from the varistor body entirely. Either of these events can, of course, lead to device failure. This has become a matter of concern to varistor manufacturers.